Field of the Invention
This disclosure pertains in general to voice coil motors. More particularly the disclosure relates to controlling a voice coil, such as stabilizing a voice coil for providing a motive force, such as stabilizing a voice coil motor. In particular this relates to voice coil motors for controlling an inspiration or expiration valve for medical ventilators, anaesthesia machines or anaesthesia monitoring.
Description of the Prior Art
To control the expiratory pressure (PEEP) a voice coil motor which controls a disc valve is a common solution. In this kind of valve the expiratory pressure is more or less proportional to the force from the voice coil. Also the force from the voice coil is ideally proportional to the current through the windings in the voice coil.
The above model is very simplified. In an actual application, the flow through the valve may vary due to the breathing pattern. This will create a movement of the disc to compensate for the varying flow. The mass of the moving parts will have an inertia which will affect the force balance in the valve. Other characteristics with the current technology are that membranes inside the valve may create spring forces and the voice coil is not ideal.
A significant drawback with this kind of valve is that it is not stable. A valve that is not stable will vibrate and consequently cause oscillations in the flow and pressure. There are several known ways to try to stabilize the valve. Using normal pressure or flow feedback to stabilize the valve is complicated since the oscillations can be fast and it is difficult to make any conclusion of the movement of the valve just by measuring the pressure or flow. Using friction is another way to prevent oscillations, but it is not desired since it may make the PEEP pressure control inferior. Other possibilities are for examples viscous damping, i.e. an opposing force proportional to the speed of the voice coil, using electromagnets or ferrofluids. A major drawback with conventional viscous damping is that the viscous damping coefficient is constant while the actual needed damping varies a lot depending on the pressure and also on the flow. The damping needed for a high pressure is several times larger than what is needed for a low pressure. The drawback with having a too high damping coefficient is that the valve responds slowly to control signals as well as to changes in the flow rate which can result in that the valve will not be able to control a constant PEEP pressure. Using a low constant viscous damping coefficient would mean that the system would be unstable for high pressures, while using a high damping coefficient would mean that the valve would respond too slowly for low pressures. Further possibilities are positional or velocity feedback by use of a positional or velocity sensor, with the drawbacks of increased cost and complexibility.
U.S. Pat. No. 5,127,400 A and U.S. Pat. No. 5,339,807 A discloses systems using a complex mechanical design wherein a separate permanent magnet is fixed to the shaft of the voice coil motor in such a way that it will move inside a separate stationary coil as a sensor for the motional EMF.
A voice coil motor may also be used for controlling other parameters than the PEEP, such as the pressure and flow during inspiration. When controlling, for example, the pressure and the flow during inspiration using a voice coil motor the system also demonstrates issues with stabilization.
It would not be straight forward to control the expiratory pressure, for example, by just changing the current to achieve a desired PEEP. Achieving a correct pressure demands an improved feedback system. Hence, a new improved design of the control of the feedback system with a better capability to avoid oscillations but still fast enough to ensure that the expiratory pressure quickly can reach the desired PEEP for different patient categories as well as different environmental parameters. It would in particularly be advantageous to have a damping that could be optimized and adjusted depending on environmental parameters, such as pressure or flow.